You can find the nude patch and nude
bases
in the files section they are free to download.
There are some nude genetics in the genetics section
a subscription is required though! Or you can make your own
or try to find them elsewhere.

First start up body
shop, create a new sim or clone 1 (i'm not
going to teach you how to do this the manual will tell you or there's other
tutorials that will elsewhere.) Select the sims genetics,
than select the skin tone.

The
custom genetics (the ones with the naughty bits in them) will appear here
you may not have all of the ones shown below that's because I have
downloaded some from elsewhere as well as created my own. The nude genetics
will have a * on
them so you can identify them. Select a genetic now you won't see it appear
on your sim 1st though...yet!

Now
select the clothing
tab for your sim and select everyday
clothing.

The
special base you
need to keep your sims nude is located here. Click on it than take a look at
your sim it should now be nude!

If
you have installed the topless
base you can find that in the TOPS
tab. Select that to just keep your sims top half topless! To use the male
and teen nude bases simply select male or teen on the left where it says age
and gender.

That's
all there is to it you can select the rest of the stuff for your sim like
makeup, hair style, face etc. As long as your sims keeps that base on in the
game they will remain nude. But if you change clothes it will disappear. You
can even buy the base at a clothes shop for your existing sims! For more
variety have each sim have a different nude genetics or have a go at
creating your own.

This
tutorial was for body shop but you can also use the create a family screen
in the game to do the exact same thing. You can also use this tutorial to
figure out how to change the genetics of your sims so they will look better
when they get undressed etc. Just don't select the nude bases for the
clothing.